The present invention relates to an active material of a positive electrode for a non-aqueous electrolyte secondary battery where lithium metal, lithium alloys, and the like are used for a negative electrode. The invention particularly relates to an active material of a positive electrode for a non-aqueous electrolyte secondary battery which is improved particularly in discharge capacity, high-efficiency discharge capacity and a maintaining rate in discharge capacity and a maintaining rate in discharge capacity of the battery. The invention also relates to a process for preparing the same and a non-aqueous electrolyte secondary battery where the noted active material of a positive electrode is used.
With the spread of portable apparatus such as portable telephones, note-sized personal computers, and so forth, the development of secondary batteries which have high energy density, are small-sized and lightweight, and have high capacity is strongly expected in recent years. As such batteries, there is a lithium ion secondary battery where lithium, lithium alloys, or carbon is used as a negative electrode and the research and development thereof is intensively carried out.
A lithium ion secondary battery where lithium-cobalt double oxide (LiCoO2) is used as an active material of a positive electrode provides a high voltage of a 4-V level and therefore, is expected as a battery having a high energy density, and put to practical use.
Latest requirements for further high capacity and high power come to need to take measures to increase the packing density of the active material of a positive electrode or to decrease the amount of an electrically conductive substance such as carbon mixed with the active material of a positive electrode, which substantially increases the amount of the active material of a positive electrode.
In general, LiCO2 is prepared by mixing certain amounts of a lithium salt, for example, such as lithium carbonate and a cobalt compound, for example, such as cobalt carbonate and calcining at temperatures of 600xc2x0 C. to 1100xc2x0 C. (Japanese Patent Laid-Open No. 304664/1989) or by mixing certain amounts of lithium carbonate and tricobalt tetraoxide having an average particle size of 2 to 25 xcexcm and calcining at temperatures of 800xc2x0 C. to 900xc2x0 C. (Japanese Patent Laid-Open No. 283144/1997.
However, conventional LiCoO2 has the disadvantages in that the packing density thereof is not increased or decrease in an amount of an electrically conductive substance leads to deterioration in discharge capacity and dependence on discharging current density.
The present inventors have come to think that this cause consists in the following. That is, LiCoO2 has a hexagonal crystal lattice and therefore crystals grow in the direction of a right angle to the C axes in synthesis to form a plate crystals. In addition, the crystals are not uniform in size and therefore mixing with an electrically conductive substance and so forth produces a number of voids, failing to raise the packing density. Furthermore, the cause for deteriorating discharge capacity or high-efficiency discharge capacity by reducing the amount of the electrically conductive substance is thought as follows. In crystals formed according to conventional processes, sintering proceeds in high-temperature calcinations, which needs strong power to pulverize the product in the production of electrodes. The pulverization produces finely divided particles to enlarge the specific areas and consequently, a large amount of an electrically conductive substance such as carbon to be added is required in order to provide electrical conductivity.
Furthermore, marked reduction in packing properties or dropping off from a current collector is found when an active material is further finely divided in order to improve highly efficient discharging characteristics to allow the flow of high power.
An object of the invention is to solve the problems of the aforesaid conventional active materials of positive electrodes and to provide an active material of a positive electrode for a non-aqueous electrolyte secondary battery which is excellent in discharge capacity, high-efficiency discharge capacity and a maintaining rate in discharge capacity and a process for preparing the same and a non-aqueous electrolyte secondary battery where the active material of a positive electrode is used.
In order to solve the aforesaid problems, the inventors have further intensively studied the size and shape of primary particles of an active material of a positive electrode and the size and shape of secondary particles formed by gathering of the primary particles, and found that the control of these factors makes it possible to obtain an active material of a positive electrode which has a high capacity and a good high-efficiency discharge capacity, thus accomplishing the present invention.
That is, the first embodiment of the invention is characterized by an active material of a positive electrode for a non-aqueous electrolyte secondary battery wherein in the active material of a positive electrode for a non-aqueous electrolyte secondary battery where lithium cobaltate represented by the formula LiCoO2 is used, the aforesaid lithium cobaltate is composed of a mixture of primary particles of small crystals which fall in the range of 0.4 to 10 xcexcm in Feret""s diameter in a projection chart by SEM observation and are 5 ∞m or less in average particle size and secondary particles formed by gathering of the small crystals which fall in the range of 4 to 30 xcexcm and in addition, the molar ratio of Co to Li is 0.97 or more and 1.03 or less. In addition, it is preferable that at least part of the small crystals constituting the aforesaid secondary particles are mutually jointed by sintering and furthermore, the aforesaid secondary particles are spherical or elliptically spherical.
Furthermore, the first embodiment of the invention is characterized by an active material of a positive electrode for a non-aqueous electrolyte secondary battery wherein the secondary particles account for 90 percent or more of particles of 9 xcexcm or more in Feret""s diameter in a projection chart by SEM observation and the volume ratio of particles of 6 xcexcm or more in Feret""s diameter in a projection chart by SEM observation is 70 percent or more in the total mixture.
The second embodiment of the invention is characterized by a process for preparing the active material of a positive electrode for a non-aqueous electrolyte secondary battery wherein in the process for preparing the active material of a positive electrode for a non-aqueous electrolyte secondary battery where lithium cobaltate represented by the formula LiCoO2 is used, the aforesaid lithium cobaltate composed of the mixture of the primary particles of small crystals which fall in the range of 0.4 to 10 xcexcm in Feret""s diameter in a projection chart by SEM observation and have an average diameter in 5 or less xcexcm and the secondary particles which are formed by gathering of a number of the aforesaid small crystals and fall in the range of 4 to 30 xcexcm where the molar ratio of Co to Li is 0.97 or more and 1.03 or less is obtained by mixing a lithium salt and a cobalt source where cobalt oxyhydroxide (CoOOH) is used as a raw material and comprises secondary particles falling in the range of 4 to 30 xcexcm and formed by gathering of a number of primary particles of 0.2 to 0.8 xcexcm and subsequently, by carrying out heat treating this mixture. In addition, it is preferable that the secondary particles of the aforesaid active material of a positive electrode are spherical or elliptically spherical and at least part of the small crystals constituting the secondary particles of the aforesaid active material of a positive electrode are mutually jointed.
The secondary particles of the aforesaid cobalt oxyhydroxide are spherical or elliptically spherical and account for 90 percent or more of particles of 9 xcexcm or more in Feret""s diameter in a projection chart by SEM observation and the volume ratio of particles of 6 xcexcm or more is 70 percent or more of the total mixture. The heat treatment of the aforesaid mixture is carried out at an oxidative atmosphere at 800xc2x0 C. to 1000xc2x0 C. for 4 to 12 hours. It is preferred to use as the cobalt source tricobalt tetraoxide obtained by heat treating the oxyhydroxide at 350xc2x0 C. to 800xc2x0 C. in an oxidative atmosphere or tricobalt tetraoxide comprising secondary particles which fall in the range of 4 to 30 xcexcm and are formed by gathering of a number of primary particles falling in the range of 0.05 to 0.8 xcexcm in Feret""s diameter in a projection chart by SEM observation.
In addition, the second embodiment of the invention is characterized by a process for preparing the active material of a positive electrode for a non-aqueous electrolyte secondary battery wherein the second particles of the aforesaid tricobalt tetraoxide are spherical or elliptically spherical.
The third embodiment of the invention is characterized by the non-aqueous electrolyte secondary battery containing as a constituent the active material of a positive electrode relating to the first embodiment.